In recent years, electric vehicles and plug-in hybrid vehicles have become widespread. For these vehicles, a motor is used. The motor operates up to high rotation using a permanent magnet for the field. Although a motor equipped with such a permanent magnet does not require an exciting current, an induced voltage generated by the field of the permanent magnet increases in proportion to a rotation speed. Because of this phenomenon, when the motor rotates at a high rotation speed equal to or higher than a certain rotation speed, the generated induced voltage exceeds an output voltage of an inverter, and a method of suppressing the output voltage of the inverter by using weak field control by PWM control is used for control at high motor rotation.
In Patent Literature 1, a motor is PWM-driven in a normal state, but when any abnormality occurs in an inverter, by switching a three phase switching element from PWM driving to three-phase short circuit driving, a DC power supply voltage can be suppressed to a predetermined voltage range.